Since the inception of the game of baseball, almost a century ago, manufacturers of baseball bats have continually sought out new materials and designs to make bats both better performing; that is, easier to hit, and/or longer hitting; and more durable; that is, less prone to breakage.
Baseball bats were initially made of wood. Today, wood baseball bats are all made of heavy and strong hardwoods, primarily ash. The rule of thumb for baseball bats made of ash (or other similar hardwoods such as hickory or birch) is that the length in inches equals the weight in ounces. Thus, today's wood baseball bats limit bat speed, and are also prone to catastrophic breakage. Such catastrophic breakage could lead to injury of not only players but also to bystanders and is a real concern to authorities. Also, as wood bats lose moisture and dry out, they lose strength and breakage increases. Replacing broken wood baseball bats is a major cost over the course of a baseball season. For these reasons, today the use of wood baseball bats is restricted mainly to major professional baseball leagues.
More recently, beginning in the mid 1970's, aluminum baseball bats captured a large majority of the market share versus wood bats, even though they are more expensive and players complain about vibrations and the “pinging” sound when a baseball is hit. There are three reasons for the success of aluminum baseball bats: 1) they are lighter than wood bats, thus increasing bat speed and increasing hitting distance; 2) they are locally less stiff than wood bats providing a “trampoline” effect upon ball impact, thus increasing hitting distance; and 3) they are less prone to breakage than wood bats.
Most recently, in an attempt to further lower the weight of aluminum bats and increase the “trampoline” effect, thinner walled and multi-walled aluminum bats have been produced, however, problems have been encountered with balls leaving dents or depressions in the bat and also, bat breakage.
Recently as well, beginning in the late 1980's, hybrid material baseball bats have been produced, incorporating polymer composite materials with both wood and aluminum. The objective of these hybrid bats is to improve either bat performance and/or durability. Such hybrid material baseball bats are described in U.S. Pat. No. 5,364,095 to Easton, U.S. Pat. No. 4,569,521 to Mueller, U.S. Pat. No. 5,395,108 to Souders, and U.S. Published Application No. 20010046910 A1 of Sutherland, all of which disclose means to improve the performance and/or durability of aluminum baseball bats by combining composite-like materials with aluminum. U.S. Pat. No. 6,139,451 to Hillerich, discloses another class of hybrid material baseball bats, which combine traditional ash wood bats reinforced full length with a fiberglass composite material, while earlier U.S. Pat. No. 3,129,003 to Mueller discloses an ash bat reinforced in the handle portion, with a composite-like material.
U.S. Pat. No. 4,014,542 to Tanikawa discloses a five-component hybrid baseball bat having a softwood balsam core, a main member made of foam, a metal tube having apertures for bonding fixed to the barrel portion only of the main member, and an outer layer of glass fiber painted with a synthetic resin.
U.S. Pat. Nos. 5,114,144, 5,458,330, and 6,152,840 to Baum disclose a hybrid multi-component bat having between five and eleven layers. Baum's bat includes external layers of wood veneer over a plurality of resin impregnated fabric socks, which in turn surround inner cores of foam, wood or aluminum which may include cavities.
The foregoing references describe hybrid material baseball bat structures, but do not disclose bats wherein at least the striking portion is constructed solely of polymer composite materials.
U.S. Pat. No. 4,848,745 to Bohannan discloses a two-dimensional filament winding process, which could be used to make an all polymer composite baseball bat, using layers (typical of today's existing composite laminate architecture) of continuous fiber reinforcement in a thermoplastic resin matrix. Bohannan describes an outer composite shell comprised of layers of helical, longitudinal, and circumferential fiber rovings impregnated in a thermoplastic resin.
U.S. Pat. No. 5,301,940 to Seki discloses a method of manufacturing a bat using a resin injection technique, with the resin being reinforced with layers of fiber.
The above two references concern possible methods for making polymer composite bats without any discussion of the fiber reinforcement architecture to be employed.
U.S. Pat. No. 5,303,917 to Uke discloses a bat comprising two telescoping tubes, made of plastic or plastic with fiber reinforcement, that overlap in the region between handle and barrel.
U.S. Pat. No. 5,395,108 to Souders discloses a synthetic wood composite bat composed of a shell of layers (or plies) of fiber-reinforced resin, a dry fiber tube inside the shell, and a rigid foam filling the shell. Souders specifically describes the existing two-dimensional fiber reinforcement architecture comprising “a plurality of cured layers of fiber resin reinforced material.” Such existing fiber reinforcement architecture, as described by Souders, is well known to perform poorly under impact loading situations, as repeatedly encountered by baseball bats. This poor performance is due to inter-laminar (that is, interlayer or inter-ply) failure between the laminates, layers, or plies of polymer composite material. Further, Souders describes an inner dry fiber tube, which is not a polymer composite material.
Moreover, polymer composite baseball bats are typically constructed using a mixture of fiber reinforcement materials such as fiberglass, graphite and aramid. Usually the mix ranges from 67% to 84% by volume of fiberglass combined with from 16% to 33% of other fibers. Generally, the reason for using a mixture of fibers is to achieve a suitable combination of weight, strength, and stiffness. The problem with such fiber reinforcement mixtures is that they tend to suffer from limited durability due to the presence of the stiffer and stronger graphite and aramid fibers, which are less durable under impact loads due to relatively low elongation under impact and relatively poor resin adhesion.
Further, all polymer composite bats of the prior art have been constructed by one of the processes commonly referred to as filament winding, tube rolling, bladder molding, compression molding, or hand lay up. All such prior art processes originated in the aerospace industry and, as such, have limitations when used to produce baseball bats at high volume and low cost.
None of the above references describe a polymer composite baseball bat wherein at least the striking portion is constructed solely of polymer composite materials having the laminate architectures or fiber reinforcement techniques required to yield a bat with the necessary combination of thickness (which affects stiffness) and durability, required to ensure the maximum “trampoline” effect, and thus good hitting performance, while at the same time being able to withstand repeated impacts without damage.
A polymer composite material consists of a non-homogenous combination of reinforcement fibers in a polymer resin matrix whereby the resultant polymer composite material has superior properties when compared to either the reinforcement fibers or the polymer resin matrix taken separately. The reinforcement fibers employed in a typical polymer composite material may be graphite (or carbon), aramid (or Kevlar″), fiberglass, or boron, or other suitable fibers, or combinations thereof The polymer resin may be any suitable resin, such as epoxy, vinyl ester, polyester, urethane, nylon, urethane, or other suitable resins, or mixtures thereof.
The following is a specific properties chart showing the density, stiffness and strength properties of various possible materials for use in making baseball bats. All data is taken from standard textbooks available in the field. Specific stiffness and specific strength are actual stiffness and strength divided by density respectively. Strengths for composite materials are given as tensile strength measured along fiber direction in a unidirectional part. Strength for wood is given as the minimum of tensile and compressive ultimate strength. Strength for metal is given as ultimate tensile strength. Unless otherwise indicated, the term “stiffness” as used in this application is equivalent to the modulus of elasticity and is a measure of the change in length of a material under loading. Stiffness or modulus is provided in pounds per square inch (usually Msi or Millions of pounds per square inch). For a tubular body, such as a baseball bat, stiffness of the material can be measured in the axial direction, parallel to the longitudinal axis of the tube or the radial or transverse direction, perpendicular to the longitudinal axis of the tube. Axial “bendina stiffness”, on the other hand, is a measure of how bendable the tube is along the axial direction. Axial bending stiffness is calculated as a multiple of the axial stiffness or modulus of the material and the second moment of area of the tube and is provided in lbs-in2. Radial “compression stiffness” is a measure of the force required to depress a section of the tube in the radial or transverse direction. Radial compression stiffness is a product of the radial stiffness or modulus of the material and the thickness and width of the tube, and is provided in pounds per inch.
DensityStiffnessSpecificStrengthSpecificMaterialslbs/ft3MsiStiffnessKsiStrengthSteel AISI 30448730.003.9085.0010.90Aluminum 6061-T616910.003.7045.0016.60Aluminum 7075-T616910.003.7083.0030.50Titanium Ti-75A28317.003.7080.0017.70High Modulus10238.0023.30165.00100.00GraphiteIntermediate10234.0019.50180.00109.80ModulusGraphiteCommercial9821.0013.30210.00132.90GraphiteE-Glass1307.003.10135.0064.30S-Glass1248.004.00155.0077.60Kevlar 498611.008.00210.00152.20White Ash422.003.008.0012.10Bigtooth Aspen271.002.304.009.30Yellow Polar291.102.404.509.80
Polymer composites are over 16 times stronger than ash and 60% stronger than aluminum. However, they are over three times heavier than ash, while approximately 20% lighter than aluminum, the aluminum bats being hollow, therefore lighter than solid composite bats, on an equal volume basis. While a solid all polymer composite baseball bat would be much stronger than either a solid ash or aluminum bat, it would be much too heavy for regular use. However, a tubular all polymer composite bat could be made both stronger and stiffer than a similar tubular aluminum or titanium bat.
In summary, polymer composite materials can theoretically be employed to manufacture baseball bats, wherein at least the striking portion is tubular and made solely of a polymer composite material, which are both stronger and stiffer than today's predominantly all aluminum tubular baseball bats. However, the two dimensional layered fiber architecture used in current polymer composite materials performs poorly under impact loading conditions such as when baseball bats are impacted by baseballs. Thus, the limited attempts, to date, to commercially produce an all polymer composite baseball bat have largely been unsuccessful, primarily due to premature bat failure or breakage. To improve durability, the wall thickness of the polymer composite tube could be increased, however, increasing wall thickness dramatically increases radial compression stiffness and weight, which in turn lowers bat performance due a decreased “trampoline” effect as the thicker bat wall springs back less after impacting the ball.
What is needed then, is a baseball bat having at least a tubular striking portion made solely of a polymer composite material with a fiber reinforcement architecture, which can withstand repeated impacts with a baseball, thus providing the required durability, while at the same time having a wall thickness thin enough to ensure hitting performance that is at least equivalent to that of the best currently existing baseball bats.
Further, what is also needed, is a high precision, high volume, and low cost process for manufacturing all polymer composite bats, resulting in desirable, and differentiated, mechanical properties in the bat handle and barrel portions required for optimal bat performance.